With the fast development in wireless communication technologies, the antenna plays an increasingly important role in various kinds of wireless communication products. Particularly, due to the tendency of developing lightweight and compact wireless communication products, the antenna size, particularly the antenna height, would decide the value of wireless communication products. However, taking the embedded mobile phone antenna as an example, while the space inside the mobile phone allowed for the antenna is much limited than ever before, the antenna still is required to cover multiband operation in order to meet the actual demands in the wireless communication system. It has been found that the loop antenna is more suitable to be internal mobile phone antenna compared to the conventional monopole antenna or PIFA antenna. This is because the loop antenna may be formed by bending and winding a thin metal wire. Unlike the conventional monopole antenna or PIFA antenna that relies on wide metal strip to increase the bandwidth, the bandwidth of the loop antenna is almost not decreased when the thin metal wire with a small wire thickness is used. Therefore, the loop antenna may have a relatively small size while achieves the same multiband operation as the conventional mobile phone antenna.
However, the lower band of the loop antenna with a greatly reduced size can cover GSM850 or GSM900 operation, but has difficulty in simultaneously covering GSM850/900 dual-band operation. Therefore, it is necessary to develop the technique for increasing the bandwidth of the loop antenna. U.S. Pat. No. 7,242,364 B2 entitled “Dual-Resonant Antenna” discloses a technique of applying a matching circuit for the internal mobile phone antenna used in the mobile communication system, so that the single-resonant mode of the antenna can have the dual-resonant characteristic to achieve the purpose of increasing the bandwidth of the antenna. However, U.S. Pat. No. 7,242,364 B2 only teaches the application of the above technique for the internal mobile phone antenna for single-band operation, but such a technique could not be directly applied to a dual-band (such as 900 and 1800 MHz) mobile phone antenna. Meanwhile, such a technique is only applicable to the mobile phone antenna having a length about a quarter-wavelength of the resonant frequency of the antenna.
To solve the above problem, a multiband folded loop antenna is developed, in which a metal strip is bent into a loop and then folded into a three-dimensional structure occupying a small volume. With respect to the operating technique of the folded loop antenna, the 0.5-wavelength resonant mode of the loop strip is used for the lower band of the antenna, and the higher-order resonant modes of the loop strip are formed into a wide upper band. Besides, a matching circuit is further used in the proposed antenna for the lower band to have a dual-resonant characteristic and increase the bandwidth. Besides, at least one tuning patch is further used in the antenna to improve the impedance matching over the upper band. With the above arrangements, the antenna is capable of providing five-band operation covering GSM850/900/1800/1900/UMTS bands and meeting the requirement of being applied to mobile phone systems.